Generally, sifters utilized within a dilute phase pneumatic conveying system, include, for instance, a pressurized container with a screen or screens of desired mesh size therein. Commonly, the container is mounted in such a manner that it is vibrated to cause the materials provided at the input of the container to move across the screen and thereby to effect the sifting action.
As shown and described by Stone in U.S. Pat. No. 5,873,469, issued Feb. 23, 1999, there is an input system disclosed to enable the prevention of unwanted contaminants from passing through the screens, or breaking up upon entering and impingining upon internal components at high velocity. Stone accomplished this by providing a tangential inlet for air and material to be screened, coupled with an external carrying system for enabling air to bypass the screening surfaces. In this way, the full diameter of the system was used for the bypass screen, and the external air loop enabled this use.
Nevertheless, the machine occupied more space with the external loop, both visually and actually, and the saving of a circular area at the center of the bypass screen was only meaningful in terms of a small percentage of the overall screening area. Even more importantly, if incoming air were enabled to pass directly through the center of the screen, the disadvantages relate to the velocity of the material impacting because of the material going through that center area at high velocity with the assistance of the air. This latter point would therefore result in the contaminants breaking up and contaminating the results of the screening.